1. Field
The following description relates to wafer reliability, and to, for example, a method for ensuring wafer level reliability (hereinafter, abbreviated as “WLR”) associated with product level reliability (hereinafter, abbreviated as “PLR”) that may be used to screen for potential problems such as “offset failure” or other reliability failure problems, during a conventional product test performed as preliminary wafer level reliability (WLR) test, thereby securing a stable level of semiconductor element development and minimizing the failure rate.
2. Description of Related Art
Wafer level reliability (WLR) test is a global standard for testing reliability of a wafer. Referring to FIG. 1, items that are generally included in the wafer level reliability (WLR) test may be grouped as element characteristic test 10, gate oxide quality test 20, and a metal wiring characteristic test 30. The element characteristic test 10 includes items such as a hot carrier injection (hereinafter, abbreviated as ‘HCl’) test 12 used for NMOS, and a negative bias temperature instability (NBTI) test 14 used for PMOS. The gate oxide quality test 20 includes items such as a gate oxide integrity (GOI) test 22 and a time dependent dielectric breakdown (TDDB) test 24. The metal wiring characteristic test item 30 includes a electron-migration (EM) test 32.
Even when a relevant element has passed a life time (L/T) specification requirement at wafer level by applying a conventional method for testing wafer level reliability, when the relevant element is placed under product level reliability (PLR) test, various reliability issues are often discovered. Theses reliability issues that are discovered during the packaging product level may slow down manufacturing or may result in high defect rate in the products.
Product level reliability (PLR) test is a reliability test performed at the packaging product level, such as high temperature operating lifetime (HTOL) test at high temperatures above about 80 degrees (for example, 125° C.). Other reliability failure includes an offset shift failure, which is a failure associated with the deterioration of characteristics of a semiconductor element. Further, additional issues with reliability that are associated with the element characteristics may be detected during the packaging product level. Many of these reliability issues cannot be detected in advance using a standardized wafer level reliability test method.
For instance, in case of an offset shift failure associated with the threshold voltage (Vth) shift in the product level reliability (PLR) test, such failure is not sufficiently closely linked with a general wafer level reliability (WLR) test method, such as HCl result in the related art). Thus, even when a relevant element has passed a reliability test in the element stage using the standard wafer level reliability (WLR) test, a lot of reliability test failures associated with the threshold voltage shift may occur during the subsequent product stage. Thus, the development of a new wafer level reliability (WLR) test capable of detecting issues associated with product level reliability (PLR) during the preliminary wafer level reliability (WLR) stage is desirable.
The HCl test 12 that are generally used involves causing an increase of threshold voltage (Vth) while electrons induced by drain induced avalanche hot carrier (DAHC) due to a horizontal field are trapped inside a gate oxide layer at the gate bias and drain interface for only an NMOS element in which electrons having higher mobility than holes are majority carriers, thereby performing the HCl test for detecting element deterioration and making a decision for pass or fail based on the specification.
However, according to the standard test, although normal HCl characteristics of a PMOS element are not assessed, test for the PMOS element is required since the effect of holes on element characteristic deterioration is large in case of a thin gate oxide and short channel device having a low ultra-thin film insulating layer (for example, less than about 30 Å). In particular, in case of a short channel device, carrier trap in the entire channel region has a significant effect on the threshold voltage shift. However, the standard wafer level reliability (WLR) test method does not take this phenomenon into consideration in determining the reliability of semiconductor elements.
Furthermore, a standard wafer level reliability (WLR) method for testing a PMOS element is the negative bias temperature instability (NBTI) test 14. The negative bias temperature instability (NBTI) test 14 is a test item for assessing the quality of a gate oxide layer at high temperatures. A carrier trap site typically located inside an oxide layer includes an interface trap charge (Qit), an oxide trap charge (Qot), a mobile charge (Qm), and a fixed charge (Qf), and holes and hydrogen trapped in Qit and Qf, which are trap sides located at the silicon and oxide interface among them, cause a failure causing an increase of the entire threshold voltage (Vth) while being diffused into the oxide layer, thereby deteriorating the element characteristics.
Accordingly, in case of an item for detecting and assessing this or a failure of threshold voltage (Vth) in the following product level reliability test, when simply assessing only a change of threshold voltage (Vth) between the gate oxide and silicon, the test is insufficient to detect a failure of threshold voltage (Vth). In particular, the shallow trench isolation that is formed to secure transistor isolation characteristics and the liner silicon nitride thin film that is applied as a material stress relaxant may cause a decrease of threshold voltage (Vth) of PMOS since there exist a lot of defect sites in which hot carriers may be trapped, thereby causing the characteristics deterioration of the element and product.
However, when the elements are assessed with only NBTI test, which is a standard wafer level reliability (WLR) test item for a PMOS element, it is possible that the potential failure associated with threshold voltage would be undetected during the element stage.